Spectroscopic properties of cool Ursa Major group members

TL;DR

This study reanalyzes spectroscopic properties of cool Ursa Major group stars to refine membership criteria, focusing on iron abundance, lithium levels, and H alpha line infilling, providing more reliable identification methods.

Contribution

It offers new spectroscopic criteria for confirming Ursa Major group membership, improving upon previous kinematic-only methods.

Findings

Ursa Major group has an average iron abundance of -0.03 dex.

Li abundance in UMa stars is higher than in Hyades but lower than Pleiades.

Cool UMa dwarfs show chromospheric infilling of H alpha line.

Abstract

Until now, most members of the Ursa Major (UMa) group of stars have been identified by means of kinematic criteria. However, in many cases kinematic criteria alone are insufficient to ascertain, whether an individual star is really a member of this group. Since photometric criteria are ineffective in the case of cool dwarf members, one must use spectroscopic criteria. Nevertheless, resulting membership criteria are inconclusive. We reanalyse spectroscopic properties of cool UMa group dwarfs. In particular, we study the distribution of iron abundance, the strength of the Li I absorption at 6708 A and the Li abundance, and the infilling of the core of the H alpha line. Twenty-five cool and northern bona-fide members are carefully selected from the literature. Homogeneously measured stellar parameters and iron abundances are given for all Sun-like stars selected, based on spectra of high resolution and high signal-to-noise ratio. In addition, we measure the Li equivalent width and abundance as well as the relative intensity of the H alpha core and the corresponding chromospheric flux. The studied stars infer an average Ursa Major group iron abundance of -0.03+-0.05 dex, which is higher by about 0.06 dex than determined elsewhere. The Li abundance derived of Ursa Major group dwarf stars is higher than in the Hyades at effective temperatures cooler than the Sun, but lower than in the younger Pleiades, a result which is independent of the exact value of the effective temperature adopted. The Sun-like and cooler dwarfs also display chromospheric infilling of the H alpha core. We present spectroscopic criteria that may be used to exclude non-members.